1. Field of the Invention
This invention relates to a method of manufacturing a fluorescent material, and more particularly to a method of manufacturing a high-luminance fluorescent material in small-size particles suitable for use in a scatter electroluminescent (EL) panel, a cathode ray tube, a vacuum fluorescent display, etc.
2. Description of the Related Art
The conventional manufacturing method for a fluorescent material to be used in a scatter EL panel (hereinafter also called the electroluminescent lamp) 7 will now be described. As shown in FIG. 4 of the accompanying drawings, the scatter EL panel 7 has a thin laminate structure in which a luminous layer 5 and a reflective insulating layer 6 are sandwiched between a transparent electrode 2 in the form of an ITO (indium tin oxide) formed on a transparent film 1 and a backing electrode 3 in the form of an aluminum foil or carbon paste. In the luminous layer 5, a fluorescent material 4 of zinc sulfide activated by copper or halogen is scattered in an organic binder. In the reflective insulating layer 6, a highly dielectric substance such as barium titanate is scattered in an organic binder. If an a. c. voltage is applied between the transparent electrode 2 and the backing electrode 3, the fluorescent material 4 emits light in an intense electric field. For its very small thickness, light weight and wide light-emitting area, this electroluminescent lamp 7 is suitable for a back light of a liquid crystal display, a planar display device and the like.
The fluorescent material 4 is manufactured usually in the following manner: Firstly, zinc sulfide (ZnS) in fine powder of several-.mu.m-particle size as a starting material, 0.1-1.0 molecular % of copper sulfate (CuSO.sub.4) as an activator, and 5-20 molecular % of halide, e.g. alkaline earth metal and alkali metal, such as magnesium chloride (MgCl.sub.2), as an coactivator and also a particle growth promoter (flux) are mixed to obtain a mixture in fine particles. This mixture is baked in a crucible in atmosphere or hydrogen sulfide at a temperature of approximately 1000.degree. C. for several hours, and then impurities such as copper sulfide on the particle surfaces are washed away with a KCN (potassium cyanide) aqueous solution, whereupon the mixture is dried to obtain a powdery fluorescent material 4.
However, this fluorescent material does not always guarantee a long life. Consequently, as disclosed in Japanese Patent Laid-Open Publication No. Sho61-296085, an improved method in an effort to manufacture a high-luminance, long-life fluorescent material was proposed. In this improved method, the above-mentioned mixture is baked at a high temperature of 1100-1200.degree. C. for 3-10 hours and is then washed with deionized water to obtain an intermediate fluorescent material, whereupon a static pressure is added to the intermediate fluorescent material by a rubber press to convert the crystal form from a hexagonal system into a cubic system. This resulting intermediate fluorescent material is further baked at 700-950.degree. C. to obtain a final fluorescent material in large-size particles whose crystal form is cubic.
Japanese Patent Laid-Open Publication No. Hei6-306355 discloses another improved method in an effort to realize a high-luminance, long-life fluorescent material. In this improved method, the intermediate fluorescent material obtained the first, high-temperature baking is stirred in a ball mill so that an impact force is added to the stirred intermediate fluorescent material to generate a strain. After crystal defects are thus caused, the resulting intermediate luminescent fluorescent is baked again in atmosphere at a relatively low temperature of 500-800.degree. C. so that the strain segregates copper to obtain a final fluorescent material.
In recent years, portable small-sized wireless devices, such as portable telephones, PHS (portable handy phone system) terminals and pagers, each using a liquid crystal display have boomed. Since its power source is a battery, every device of this type essentially requires a low consumption. In particular, an electroluminescent lamp to be used as a back light of the liquid crystal display is relatively high in consumption as compared to other components of the device, and consequently, low-operating-voltage and high-luminance electroluminescent lamps are demanded. However, in an electroluminescent lamp using the conventional fluorescent material manufactured in the above-mentioned method, the operating voltage is an a. c. voltage of approximately 50 V or higher and is hence larger in consumption. Further, the larger the size of an inverter for converting the d. c. voltage of a several-V battery into an a. c. voltage, the higher the price.
One of reasons for the high operating voltage is that the medium particle size of the luminescent material 4 according to the conventional method is about 20-30 .mu.m, which is large. Specifically, in order to uniformly print the luminous layer 5 in which this fluorescent material 4 is scattered in an organic binder, the thickness of the luminous layer 5 would be approximately 50 .mu.m so that no effective application of voltage to the luminous layer can be achieved. This decreases the intensity of electric field on the fluorescent material to give only an insufficient luminance; therefore, the operating voltage must be increased high enough to obtain an required luminance. If the fluorescent material were high in luminance and small in particle size, the thickness of the luminous layer could have been reduced to lower the operating voltage of the electroluminescent lamp. Further, assuming that the luminance is sufficiently high even though the particle size is large, it is theoretically possible to lower the operating voltage, which is practically difficult to realize. It is also theoretically possible to reduce the particle size by lowering the baking temperature, shortening the baking time or reducing the amount of flux. Any of these attempts would not be practical as it actually lowers the luminance; especially, reduction of the amount of flux would result in an inadequate coactivator to lower the luminance as flux serves also as a coactivator.